Conventionally, the adjustment of conical pulleys is effected by means of hydraulic, axially-acting tensioning means which are apportioned pressure medium by a pump via a control valve in order to set and maintain the transmission ratio.
There are however also continuously variable conical pulley transmissions which are adjusted mechanically.
Here, the chain-type converter has a fixed ratio range which is dependent in particular on the length of the traction mechanism and also on the maximum opening extent of a disk set. The maximum opening extent of a disk set is assumed when the axially movable conical pulley is opened to the maximum degree. The maximum opening extent is determined here by means of a stop. On account of the geometric relationships, the minimum running radius of the traction mechanism and therefore ultimately the extreme transmission ratios of the chain-type converter transmission are co-determined by the maximum opening extent of a disk set. Here, the maximum opening extent of the drive output disk set determines the overdrive transmission ratio, while the maximum opening extent of the drive input disk set determines the underdrive transmission ratio.
A mechanical delimitation of the ratio range of a corresponding conical pulley transmission is thus possible by means of the precise positioning of the stops against which the movable disks abut at their maximum opening extent. Since the angle of the conical pulleys is approximately 10°, the corresponding minimum and maximum running circle varies with a ratio of 1:3 in the event of a movement of the stop. It can therefore be seen that there is a very high requirement for the stop to be mounted with a high degree of positioning accuracy.
The running circle is however also influenced in the same way by the width of the traction mechanism. In the case of a chain, as is disclosed for example by DE-A 1 294 130 or DE patent 28 48 167, said width corresponds to the length of corresponding rocker pins via which the transmission of force from the traction mechanism belt to the conical pulleys takes place.
The width of the traction mechanism belt is subject to variation on account of wear of said rocker pins, such that the running circle can likewise vary corresponding to the above-described geometrical relationships.
Here, the change in the running circle associated with the wear of the traction mechanism belt moves the entire ratio range of the conical pulley transmission in the direction of underdrive. Here, however, the lowest possible underdrive transmission at the same time becomes smaller (transmission ratio=1/i=n2/n1)
Electronic transmission ratio regulators which are provided nowadays are duly capable of predefining the starting transmission ratio as a setpoint value upon start-up of a corresponding conical pulley transmission. A corresponding setting of said starting transmission ratio is however only possible once the transmission rotates and the actual value is thus available from corresponding rotational speed signals of the drive input and drive output shafts. In practice, this has the result that the first rotations of a conical pulley transmission until the intervention of the electronic transmission ratio regulation take place initially at the mechanical end transmission ratio.
It may therefore be the case that, in transmissions which intentionally have a ratio range restriction, the transmission operates in an explicitly undesired range.
In specific applications of conical pulley transmissions, for example in the case of a power split in a combination of planetary gear sets which permit an expansion of the ratio range, the requirement for precise positioning is significantly greater still, otherwise transmission ratios and/or speeds are reached which are not desired or not permitted.
To prevent this, the conical pulley transmissions are always produced with a considerable wear allowance, such that the operating point of the transmission combination remains in a permissible range even at the end of the service life of the traction mechanism belt.
Aside from the fact that the possible ratio range of the conical pulley transmission is considerably restricted in this way, there remains the risk that the transmission combination could nevertheless move into an undesired operating range in the event of unexpectedly high wear.